If I use UploadSiteHostedPictures to upload my pictures to eBay Picture Servers, for how long are the pictures retained?
Pictures that are uploaded to eBay Picture Servers need to be associated with an item, otherwise they are removed after the date returned in <UseByDate> in the response . They need to be used in the PictureDetails container of AddItem / ReviseItem / RelistItem family of calls. If you use the pictures just in the Item.Description field, but not in Item.PictureDetails.PictureURL or Item.PictureDetails.GalleryURL, the pictures will be removed.
If the image is used in a listing before the date specified in the <UseByDate>, then the expiration will extend to the associated items sale_end date plus 90 days.